Nonvolatile programmable memories operate similar to read only memories with the attribute of being programmable at least once. Included in nonvolatile memory types are Flash EPROMs (ultraviolet erasable programmable read only memories). Typical Flash EPROM cell structures are either charged or discharged in order to program or erase the cells and store information. In general terms, charging refers to the activity of putting electrons onto a floating gate of the cell, while discharging refers to the activity of taking electrons off of the floating gate of the cell. When charged, the cell has a low current and a high threshold voltage, V.sub.t. Conversely, when discharged, the cell has a high current and a low V.sub.t. A typical charged V.sub.t, is usually 5V, and a typical discharged V.sub.t is usually 2V. In addition, in general, the ultraviolet threshold voltage (UVV.sub.t), which is used to define the VV.sub.t, of the device when the floating gate does not have any charge, i.e., when in a charge neutral state, is also about 2V, making the discharged state and the UV erased state the same.
Currently, the UVV.sub.t is determined in large part by the doping of the core cell channel. However, the doping of the channel is also dependent on the need to control problems due to short channel effect. The short channel effect generally refers to the problem associated with drain induced barrier lowering (DIBL) and threshold voltage roll-off with channel length. Presently, scaling down of Flash EPROM cells has been considered critical in continuing the trend toward higher device density. As the size of the cells shrink, the short channel effect becomes problematic. With larger cell sizes, the channel between the source and drain is long and the transient electrical field generated is very small during the charging period, i.e., during the period when the voltage is applied to the source. However, as the channel becomes shorter due to cell size shrinkage, short channel effect becomes significant.
While altering the doping in the channel is typically utilized to counteract the short channel effect as device sizes shrink, unfortunately, changes in the doping in the channel may adversely change the threshold voltage. Too high of a doping level not only can make the short channel effect worse but can also translate to a higher threshold voltage for the cell. Thus, optimized channel doping levels for combatting short channel effect often produce less than optimal threshold voltage values, and vice versa.
Accordingly, a need exists for a manner of providing a flash EPROM cell that has greater flexibility in optimizing core channel implant for short channel effect while maintaining desired threshold voltage.